Radio communication systems generally include a message transmission device, such as a base station, for encoding messages using a signalling protocol such as FLEX.TM.. The encoded messages are then transmitted as a radio signal to portable devices, such as paging receivers or transceivers. To increase the probability that a pager receives an error-free message, some systems employ "time diversity", e.g., the transmission of duplicate messages.
Conventionally, different types of TD systems exist. Some TD systems involve the reception of duplicate messages by a pager, which then determines which of the received messages includes the fewest number of errors. The least erroneous message is stored and displayed to the user. Other systems provide for codeword combining, in which error-free codewords of messages and perhaps even addresses are selected from each of the duplicate transmissions then recombined to form a correct message in its entirety. Bit combination time diversity (BCTD) is the most accurate TD method. When BCTD is utilized, the pager receives duplicate transmissions and selects therefrom correct bits from each of the transmissions. The selected bits are then recombined to form a complete, error-free message. The drawback of BCTD is that each duplicate transmission must be located in a predetermined signal location so that the pager can calculate, such as from a fixed reference point, the exact locations of the bits of the duplicate transmissions.
In one BCTD system that employs the FLEX.TM. signalling protocol, the number of words in each frame is divided evenly according to the number of duplicate transmissions. In the conventional FLEX.TM. protocol the radio signal is divided into cycles of one-hundred-twenty-eight (128) frames, each including a predetermined synchronization pattern (sync) and a plurality of words. When the radio signal is to be transmitted at 1600 bits per second, for example, eighty-eight words are included in each FLEX.TM. frame. These eighty-eight words are usually divided into an address field, which includes addresses of pagers to which messages are being transmitted, a vector field following the address field, and a message field. Some addresses, such as those referring to numeric or alphanumeric messages, refer to a vector contained in the vector field, which specifies an area within the message field wherein a corresponding message is located.
In a BCTD system, when a message is to be transmitted two times, for instance, each frame is divided into two segments of forty-four words that follow the sync pattern. New message information is provided in the first forty-four word segment of each frame, while duplicate information from the preceding frame is always transmitted in the second forty-four word segment of each frame, as depicted in the signal diagrams of FIGS. 1 and 2. In FIG. 1, new information is provided in the first forty-four words of frame x. FIG. 2 illustrates the following frame, i.e., frame x+1, in which new information is provided in the first forty-four words and duplicate information of that provided in frame x is transmitted in the second forty-four words. Pagers that receive duplicate messages are able to exactly determine the location of each bit of a duplicate message because the duplicate messages are always provided in the same order within a second forty-four word segment of the subsequent frame. Therefore, a pager can select correct bits from both the new information, which is included in a first frame, and the duplicate information, which is included in a second frame, then recombine the bits to form an entire correct message.
However, this approach to TD results in a 50% decrease in capacity because a predetermined half of each frame is always dedicated to the transmission of duplicate information. When two or more repeat messages are transmitted, the situation worsens. For example, when four transmissions of each message are provided, each frame is divided into four even segments. New information is transmitted within the first segment of each frame, while the remaining three segments are dedicated to the transmission of duplicate information. Therefore, such a system results in a 75% decrease in capacity from a non-TD FLEX.TM. system.
A further problem is that, in the described BCTD system, only pre-programmed pagers can receive the duplicate messages. As mentioned above, FLEX.TM. conventionally includes an address field followed sequentially by vector and message fields for each frame. In the prior art BCTD system, though, the address, vector, and message fields associated with duplicate transmissions are positioned within the inner regions of the frame separate from fields associated with original transmissions. Only pre-programmed pagers therefore know to look in the middle of the frames for the duplicate transmissions. Other pagers, such as those that may roam into the system, will miss the duplicate transmissions because the roaming pagers only expect to find, for each frame, a single address field followed by one vector field and one message field.
The conventional prior art BCTD system is further problematic in that the limited number of words dedicated to original transmissions can result in the delayed transmission of lengthy messages or the fragmentation of lengthy messages into smaller portions for transmission. In either situation, the subscriber must wait a longer time than usual for message reception because his pager is waiting for the entire transmission of the message or for a later-transmitted portion of the long message. As a result, subscribers can be inconvenienced by the delay of message reception.
In FLEX.TM., messages transmitted to each pager are generally sequentially numbered so that, when a message is missed, the pager subscriber can call into the base station and request the re-transmission of the message that was missed. In the prior art BCTD system, though, the pager can receive a new message before a message having a previous number has been correctly "built." For example, the pager could erroneously receive a first transmission of message 1, correctly receive a first transmission of message 2, then receive the duplicate transmission of message 1. In this case, the pager would conclude that message 1 had been missed because message 2 was received first. Therefore, the prior art BCTD system can cause pagers to erroneously determine that messages have been missed.
Thus, what is needed is a method and apparatus for sending duplicate transmissions without decreasing system capacity by a predetermined, fixed amount and without significantly decreasing battery life. Furthermore, the method and apparatus should provide for reception of duplicate transmissions by roaming pagers. The method and apparatus should also provide for the transmission of duplicate messages without unreasonable delay and should not result in missed messages when the system operates on a "numbered messaging" scheme.